Natural refuge for two-gene Bt cotton meets resistance

A proposal to eliminate the structured, non-Bt refuge requirement for Bollgard II cotton technology for farmers in Texas, the Mid-South and the Southeast has met resistance in Louisiana.

Wisner, La., crop consultant and producer Ray Young told an EPA scientific advisory panel in June that he prefers keeping the current non-Bt refuge in place for Bollgard II cotton, saying it's a small price to pay to protect a very valuable technology.

“If there is a slight chance that (resistance) could be a problem, then we ought not do it,” Young said of the proposal. “They say there is enough natural refuge out there that with Bollgard II, we basically won't ever get resistance. I say it will happen sooner or later. We haven't had an insecticide yet that budworms have not gotten resistant to.”

Monsanto researchers and some university scientists are saying that non-Bt cotton refuges required to prevent target pests from developing resistance over time are not necessary in the two-gene system of the Bollgard II technology.

Some researchers also say that susceptible insects needed to delay resistance may come from a large number of alternate crop and native host plants, which would also appear to minimize the need for a non-Bt refuge in Bt cotton.

The EPA is expected to make a decision on the proposal for Bollgard II cotton in September. The decision will not affect the refuge requirement for single-gene Bollgard cotton, which is registered to be commercially available through the 2009 season.

Young says other Louisiana cotton producers are in agreement with him. “We don't need to lose this technology. I'd hate to think what would happen if we had to go back to spraying with tobacco budworm treatments.”

“You do sometimes lose a little bit of yield (with a non-Bt cotton refuge), you have to grant that, and it is a little bit of trouble. It's just that the long-term benefit is worth the short-term problems you might have. I don't think there is any question that we are delaying resistance (with a structured non-Bt refuge).”

Over the last four years, Monsanto and cooperating scientists conducted tests in six states using carbon isotope and gossypol analyses to determine larval food sources for tobacco budworm and cotton bollworm.

Scientists have known bollworms can complete larval development on a number of plants, including corn, soybeans and grain sorghum. They believed the number of hosts for tobacco budworm were limited, but the recent data indicates that tobacco budworms are likely developing on a number of other plants.

Tests in Georgia and North Carolina showed that 80 to 90 percent of the tobacco budworms found in those states were “non-cotton” moths; that is, they had fed as larvae on plants other than cotton.

But in the Mid-South and southeast Texas, where growers tend to plant fewer crops besides cotton, the percentages of non-cotton tobacco budworm moths weren't as high. And that bothers Young.

“North Carolina has a lot different cropping system than we have,” Young said. “They don't have the big cotton fields usually. They have a lot of other crops and they have a lot of alternate hosts. The farther you go west, the less of that alternate host you have. We don't have hardly any alternate hosts for tobacco budworms. Cotton bollworms, we do.”

Young is also concerned that during years of drought, weedy hosts for the worms may die, reducing the size of, or eliminating the natural refuge. Where is the refuge in the middle off a 1,000-acre cotton field if you don't plant one?”

Walt Mullins, Monsanto's technical manager for Bollgard and Bollgard II, said there is a “clear difference in the amount of natural refuge” between North Carolina/Georgia and the southern Delta.

“We expected this to be the case. But anytime we caught 10 moths in a trap, there was never a case where there were zero moths coming from a natural refuge. There was always a measurable level of contribution from non-cotton hosts.”

Mullins noted that despite the smaller percentage of moths coming from non-cotton hosts in the southern Mid-South region, “the question is how much do you need to maintain durability. There is no absolute way to determine that. But according to models that EPA has been using — adopted for a two-gene system — something less than a 0.05 percent would be adequate.

“The model is telling you that even when you have 1 percent coming from non-cotton hosts, you still have 20 times what you need. We have lower numbers of tobacco budworms coming from a natural refuge in the Mid-South, but when you consider what is actually needed based on the two-gene technology, you need very little contribution to make this thing work.”

Mullins says scientists have not documented a case of surviving budworms on single-gene Bollgard cotton. “The dose may be so high that resistance is going to be extremely rare or a remote chance. Then when you put two genes that are both independently extremely high dose, it makes the risk of a fully resistant budworm to Cry 1 and Cry 2 that much more remote.

“The industry is not walking away from the structured refuge, Mullins said. “We're adding a second gene to improve the inherent resistance management potential of the product. We've done that with Bollgard II and picked up some additional control as well. I think that demonstrates that the industry is still committed to resistance management.”

Mullins noted that a natural refuge allows Monsanto to sell more Bt cotton, since cotton producers could conceivably plant all their acres in it. “This is a win-win-win situation. It's a win for the environment, because there will be less pesticide used. It's a win for the grower because he doesn't have the cost or complications associated with planting a small field of non-Bt cotton.

“And it's also a win for the seed industry because it addresses the issue of farmer compliance with insect resistance management rules.”

Mullins noted that from a resistance management standpoint, “we are better off with Bollgard II and a natural refuge system than with Bollgard and the current structured refuge system.”

While Young acknowledges that the chance of resistance developing in two genes is remote, “I don't want to deal even with the possibility, let alone the probability. Having two insecticides in Bollgard II is good, but one of them has been in there for 10 years, so one of them is partly used up. It's not like there are two brand new ones.”

LSU AgCenter Extension entomologist Roger Leonard is still “on the fence” as to whether the industry should go to a natural refuge for Bollgard II cotton. “I don't have enough confidence in the data that exists to support the concept for the Mid-South at this time.

“If enough data is collected to support this change, and the data is similar to that collected across the Atlantic Coast states, then I would be more willing to believe that we would not be compromising the durability of this technology.

“Across the Mid-South, there are a number of variances in the data which show a high percentage of adults are being produced on non-Bt cotton. Those data are very important and need to be explained, both spatially and temporally.

“The big concern for the Mid-South is what is happening in late July, August and early September,” Leonard said. “What are the plants tobacco budworms are developing on that contributes to that population coming from non-Bt cotton? There are instances where higher than 90 percent is being produced on non-Bt cotton.”

How many budworms should be coming out of a natural refuge “is the million-dollar question,” Leonard said. “I don't know, and I do not believe other scientists can answer this question either. For my own satisfaction, I'm looking for consistency in the data and biological explanations on the variation in that data.

“For example, if resistance occurs, the initial occurrences are not widespread, it's happening across a selected area. I contend that there are areas where a tobacco budworm population could constantly develop on non-Bt cotton with little or no alternate host contribution. All the data should be reviewed very carefully before making such a change.”

Leonard added that the studies by Monsanto and university researchers “has done more to advance the study of Heliothines ecology in U.S. cropping systems than anything we've done in the last decade. I'm very supportive of the previous work and current studies. But at this time, I'm not in agreement with all interpretations of the data and some of the conclusions.

“This may be one of the most important decisions the cotton industry has addressed during this decade. If we make a mistake with this technology in the current production systems, producers may not be able to adapt to the loss of this critically important pest management tool. I, for one, just want to be sure that we are not endangering the cotton industry by sacrificing long-term sustainability with short-term economic gains.”

The National Cotton Council's Environmental Task Force released a report in May 2006 which supported a natural refuge for Bollgard II in all production regions in the Southeast, Mid-South, Texas, Oklahoma and Kansas, “while at the same time recognizing that the data are variable across regions.”

Dow AgroSciences is also in agreement with the proposal for a natural refuge. “Based on our modeling, and considering the approach taken to estimate the non-cotton contribution to the tobacco budworm refuge to be reasonable, Dow AgroSciences agrees that structured refuges are unnecessary to ensure the durability of two-gene Bt cotton, including Bollgard II and WideStrike,” said Duane Canfield, cotton market specialist for PhytoGen and WideStrike.